This invention relates in general to cardiac assist systems and more particularly to an extracorporeal system employing a gravity filled disposable blood pump automatically controlled to continuously optimize blood flow asynchronously. Cardiac assist systems are employed in a number of clinical situations including post cardiotomy ventricular dysfunction, post myocardial infarct, cardiogenic shock, as well as a bridge to heart transplant. Many of these systems are, at best, semi-automatic in that they require adjustment of pump controlled variables such as beat rate, systolic duration, drive pressure, vacuum pressure, flow rate, and timing in order to assure proper blood flow in response to changing conditions in the patient.
One such system is described in German Application No. 2658104 filed Dec. 22, 1976 and published Jan. 12, 1977. That reference describes a system employing a diaphragm pump with a diaphragm position transducer providing a signal indicating when the diaphragm is fully collapsed or fully extended. This transducer signal is used to operate an electromagnetic valve which either allows the pneumatic flow to move toward the diaphragm, tending to collapse the wall and thus provide a systolic pumping action or to decrease pressure allowing blood to fill the pump. The system system is arranged so that the pump always pumps at the lowest possible frequency. Thus a higher filling rate at the input causes the pump to beat faster.
A second prior art method is described in USSR inventor's Certificate No. 733688 filed Nov. 23, 1977 and published May 15, 1980 entitled "Device For Indirect Control Of Performance Of Artificial Heart With External Pneumatic Drive". This certificate describes a system for generating a signal controlling the rate of change of gas pressure and generating a signal correcting for the compressibility factor of the gas in a pneumatically driven heart pump. An algebraic summer sums the signal indicative of volume flow rate together with a signal indicative of the derivative value of gas pressure, thereby producing a corrected signal of flow rate. To determine the stroke volume of the artificial heart, a filtered signal from the summer is transferred to an integrator, the integration period of which is controlled by a specific control unit.
Other systems are described, for example, in the book "Assisted Circulation" edited by Felix Unger, Innsbruck, published by Springer-Verlag in 1979. At page 322 of that volume there is described a method of measuring pressure and flow in which a signal indicating the change of direction of the air flow and one indicating the arrival of the air flow at a zero value are generated. The two signals are combined and analyzed to produce values for the stroke volume in air, the heart frequency, the systolic duration, the difference of systolic and diastolic pressures, the center of the area of systolic flow versus time and the center of the area of diastolic flow versus time. The real stroke volume of the pump can then be calculated with a minicomputer whose value is fed back to the surgeon.
One of the disadvantages of the prior art systems is the possibility of hemolysis of the blood cells due to an axisymmetric bladder collapsing too far during the systolic beat and thus physically compressing the blood between its walls. Additionally problems are introduced whenever the pump uses vacuum, as do many of the prior art pumps, since this can lead to collapse of the natural atrium or suction of air.